Signaling system



L. u 'a Recanati? A TTORNEY /NVENTOR H. N YOU/S T Nov. 18, 1941. H. NYQulsT SIGNALINQSYSTEM Filed sept. l, 1939 F/G.l

INPUT Patented Nov. 18, 1.941

SIGNALING SYSTEM Harry Nyquist, Millburn; N. J., assigner to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 16, 1939, Serial No. 295,237 14 claims. ,tot 17a-6.6)

This invention relates to telegraph signaling both for usual telegraphy and for some forms of picture transmission or television by a series of marking and spacing intervals. In such signaling the problem of distortion as represented by departure from correct ratio of marking and spacing intervals has received much consideration and substantial improvements have been made as exemplified in such patents as that to Herman 1,886,808, November 8, 1932. To some extent, however, the problem still remains, especially with some particular types of telegraph systems.

This invention relates more specifically to socalled single side-band carrier frequency signaling systems and to methods and means for reducing certain distortion effects therein. In carrier frequency telegraph signaling it is common to intermittently impress the output of a carrier frequency source on a transmission line by means of a key or equivalent device, the duration of the trains of waves on the line corresponding more or less closely to the keying motion. The process associated with the closing and interrupting of the circuit, however, is not so simple as that of merely introducing on the line the carrier frequency alone but it gives rise to `a band of higher frequencies adjacent to the carrier and a band of lower frequencies, the two constituting an upper and a lower side-band as modulation products. Since the signaling information in the one side-band is the same as that in the other sideband, it is possible to eliminate one of these and transmit the other only, thus reducing the width of the frequency band required for signaling. Also, the carrier itself may be eliminated to such extent as may be desired, and reintroduced at the receiving end for demodulation purposes, sometimes called homodyne reception.

My invention relates to a system in which a filter is used to eliminate one of the side-bands and to substantially reduce the carrier but no provision is made for the reintroduction of the carrier at the receiving end.

In an article published by me entitled Certain topics in telegraph transmission theory, appearing in the Transactions of the American Institute of Electrical Engineers, Vol. 47, April 1928, page 617, I have called attention to the fact that when a filter is used to pass one side-band only, the output wave of the filter may be considered as made up of two components which I there call the in-phase component and the quadrature component. Referring to the frequencies in the side-bands, the in-phase component may, for the purposes of this disclosure, be defined as one whose zeros coincide with the zeros of the steady state carrier frequency. Its envelope is substantially identical with the total wave received in double side-band transmission and by properly designing the filters it is possible to make this component distortionless in the sense in which that term is used here. The quadrature component is one whose zeros are a quarter period out of phase with the zeros of the steady state carrier frequency. This component is present when the in-phase component is changing in amplitude and tends to have its maximum value when the in-phase component is changing most rapidly.. When the steady state is reached it falls to Zero. The quadrature component modies the shape of the received wave and when single side-band is used will, in general, introduce some bias at the receiving end; this being in addition to such other biasing effects as may be present and which would presumably be compensated for. The purpose of my invention is to compensate for the type of bias due to this quadrature component.

The invention will be better understood by reference to the following specification and the accompanying drawing in which Fig. 1 shows a telegraph circuit adapted for single side-band transmission, with certain bias compensating features;

Figs. 2A to 3C are explanatory of certain aspects of my invention;

Fig. 4' is a form of circuit in which more complete compensation is made for the quadrature component;

Fig. 5 shows the application of the principles of my invention to certain forms of telephotography or television;

Figs. 6 and 7 are curves explanatory of certain aspects of my invention; and

Figs. 8 and 9 relate to the application to certain other forms of telephotography.

Referring vmore particularly to Fig. 1, there is shown a sender T subject to signal waves of direct current corresponding to the signal elements to be transmitted. The sender T controls the output of a carrier frequency generator G1 as a result of which there is impressed on the line a succession of marking and spacing intervals of the carrier frequency. A single side-band lter Fi is introduced in the line passing the upper side-band or the lower side-band and in any case with its cut-off at or near the carrier frequency fc. At the receiving end this signal is detected by the detector D1, the output of which operates this will be assumed distortionless in the sense lin y which that term is used herein. Fig. 2C represents qualitatively the accompanying envelope of the quadrature component which, as pointedV out above, has the property of lbeing present only when a change is occurring, reducing "Lto lZero The best value for the resistance R in any particular case can best be determinedby trial, using any standard or well-known type of bias measuring circuit and adjusting the value of resistance R until minimum bias is obtained. One known type of bias measuring circuit is disclosed in a paper by Nyquist, Shanck and Cory, published in the Journal of the American Institute of Electrical Engineers, `March, 1927, pages 231 to 240, particularly Fig. '7 and its accompanying description.

VWhile compensation for the quadrature component indicated in Fig. 1 is useful, further rewhen the transmitted current has been-held^con stant for some time. In the absence .of horno-y dyne reception the envelope of the received wave is obtained by combining the curves of Figs. 2B and 2C by root-mean-square addition. The result is shown'in Fig. 2D. Since the laddition of two components in quadrature to each other always Agives a resultant which is greater'r than either, the presence of the quadrature component results in a bias indicated in Fig. l2D where the dotted line is a reproduction of a curve of Fig. 2B. v

Inspection of Fig. 2D suggests 'the need of a correcting constant bias by the useof a constant biasing current on the receiving relay or the transmitting relay, or' an auxiliary relay in tandem with either. The bias determined lfrom Fig. 2D is not, however, the correct one for all cases, as will `be'apparent from a study of the following figures. n f l Fig.'3A represents a short transmitted mark and Fig. 3B representsthe corresponding received distortionless iii-phase component. This component is distortionless in the sense that ii the receiving relay is appropriately adjusted, it will' open and vcloseat points corresponding to M and N of Fig. v3B and the marking intervalwill be the same as indicated in Fig. 3A. The correvsponding quadrature component is shown in Fig. 3C and is the sum of'two curves such as shown in Fig. 2C displaced from eachother by the 'duration of the marking'interval and being of opposite sign. vThe result is that there is mutual interference between them, the extent of the interference being in proportion to the shortness of the marking interval. It is evident that as a result of the combining of Figs. 3B and 3C byl root-mean-square addition the vbias vonl the resultant envelope will vbe less on a short mark than on a long mark. Further, it is evident that in general the bias caused by the quadrature com-,-v ponent is a function of what has gone before, at least in large measure. Still further,.it Ybecomes evident thatif a constant correcting biasis introduced it should not be made equal to that deduced from Fig. '2C but should be made equal to a representativeor average value. Moreover, it; becomes evident that for the best results the bias should be made variable and be made a function'ofthe preceding signals. l

AReferringagainto Fig. 1, 'the circuit there shown vis one in which the average bias due to the quadrature component is compensated. As already stated, T is the transmitting relay which sends unbiased signals, F1 is a single side-bandy filter,`D a detector and RR a receiving relay.

R isa resistance in a constant biasing circuit sov finement is obtained by compensating for variations .inbias as well as for the average. A circuit for this purpose is shown in Fig. 4. The transmitting vrelay rT sends out unbiased signals as before but the contacts are so arranged that the signals are turned over, i. e., marking intervals become spacing intervals and the reverse. The output of the single side-band lter F1 is detected Lat D1 and is in turn used to control a second oscillator G2 preferably of the same frequency as G1. The output of the generator .G2 is passed through a second single sideband filter F2 substantially the same in form as F1. It will be noted, however, that the .intermediate relay IR is so connected as to give an additional turnover to the signal so that in the signals as received from relay RR marking intervals correspend to marking intervals as impressed on T. The quadrature component resulting from filter F1 causes the signal at the input of the intermediate relay IR to be biased to marking but by virtue of the turnovei' for the contacts of relay IR this will appear as a spacing bias and this spacing Vbias will .balance out the tendency toward marking bias due to filter F2. It will be desirable by means of a resistance R to adjust the bias Y of the relay IR to take out or compensate for the averagejbias. The residual variations are then impressed on the transmitted signals and will compensate for residual variations due to F2. An'alternative arrangement is to introducefno bias in the relay lat the receiving station but to introduce a bias in the sendingrelay T of such valuejas to make it cancel the average bias introduced at IR.

' From the `above it is seen that, speaking broad-` ly, (1), the .rst single side-band filter is introduced to undo the biasV introduced by the second; (2) relay 'IR is preferably biased so'that the two filters will work on nearly similar signals (except y of the,y circuit. Thus the line might come between y D1andIR or between F1 and D1, the over-all electrical effects being substantially the same.

In some systems for transmitting half-tone pic-- tures a steadyy stream of reversals, using double side-band carrier frequency, are transmitted, a neutral tone corresponding to equalmarking Aand spacing intervals. The voutput of the picture transmitting machine is used to vary the bias of these signals in accordance with the shade tol transmission and incorporating .the principles ofY compensation forqua'dra'ture component is illus,-

trated in the circuit of Fig. 5. In this figure the rotating drum I carries the picture to be transmitted. Light passing therethrough to va photoelectric cell E gives rise to a current in the photoelectric circuit, the magnitude of which varies in accordance with the shade at the successive points on the picture. A generator G4 maintains the reed of the relay R4 in continual vibration, alternately making and breaking contact for the circuit of generator G5 of any desired carrier frequency. With the current in the photoelectric cell corresponding to an arbitrary neutral tone, the relay R4 is adjusted so that the marking intervals are equal to the spacing intervals. As photoelectric current flowing through the coil C4 varies in amplitude from the arbitrarily chosen neutral value, the marking to spacing ratio of relay R4 is correspondingly altered. In order to operate the system on a single side-band the filter F4 is introduced but with it there appears the quadrature component referred to above. Accordingly, there is introduced in the circuit the detector D4 and relay which operates to turn over the signal and to then control a second generator Gv preferably of the same frequency as G5, whereupon the signal is transmitted through the lter F5 to the receiving station. At the receiving station a second reversal is made by the detector and relay R5. The Wave that is nally received is, therefore, compensated for the quadrature component introduced incidental to the single side-band transmission and the light valve L. V. registers the picture on the revolving drum in the form of a half tone picture as previously stated.

As a modication of the operation of Fig. 5 the intervening relay R5 may be omitted and the signal is then eifectively turned over by the addition of direct current after detection.

If the system of Fig. 5 is operated Without the lters, that is, if it is operated on the basis of the double side-bands, then the system from the transmitting relay R4 to the receiving relay is equivalent to a transmitter whose input-output characteristic is shown in Fig. 6, this being for the case where the transmission system itself introduces no bias. In Fig. 6 the input represents numerically the shade of gray of the transmitted picture and the output that of the received picture. Fig. 7 shows the corresponding characteristic with a single side-band system having the quadrature component uncompensated. As an alternative to the circuit arrangement of Fig. 5, it is possible to introduce a non-linear transducer ahead of the transmitting relay, as shown by 0 of Fig. 5. This transducer should have the inverse characteristic of that shown in Fig. 7 so that when this non-linear element is worked in tandem with the remainder of the transmission system, the resultant -characteristic is linear, as in Fig. 6.

Still a further application of the principles of my invention to telephotography is illustrated by Figs. 8 and 9. Referring to Fig. 9 there is shown a means Aat ll) for producing a direct current of variable magnitude in accordance with the characteristics of a picture to be transmitted. This device may well be identically the same as that shown in Fig. 5 and a portion of the output curve of the device l0 is shown by the full curve of Fig. 8. Using the line at as the base line, the ordinate of the curve may be taken as proportional Ato the intensity or degree of whiteness of the successive points on the picture. In some systems of telephotography it has been common to use this variable direct current to modulate a high frequency kwith two side-bands. This high frequency signal with its two side-bands is then transmitted to the receiving station where it is directly detected and used to control some mechanism such as a light valve for the purpose of reproduction' of the picture. In my system, however, I propose to transmit one side-band only. To that end there is shown in Fig. 9 a modulator circuit M1 supplied with an oscillator of a convenient frequency f1. The output of the modulator passes through the single side-band lter F1, one edge of which lter is at or near the frequency f1.

As pointed out above, if the output of this filter were now directly transmitted and detected at the receiving station there would be distortion due to the quadrature component. Quite aside from any other distortions which might be present in the transmission system the form of the wave received would depart from the full curve of Fig. `8 in a manner indicated by the dotted curve, the distortion corresponding or being analogous to the marking bias introduced because of the quadrature component, as described in connection with the case of telegraphy. In order to compensate for this distortion I follow the same procedure as described above, namely, the output of the lter F1 is detected in the detector D1, which would give rise to a current represented by the dotted curve of Fig. 8.' A turnover is now accomplished in a manner to give rise to a Acurrent the reverse of that which came from the amplifier A. This is conveniently accomplished by subtracting what would be the output from the detector D1 from a direct constant current. This again is illustrated in Fig. 8 in which a current of magnitude ab is supplied by a battery Il in such a direction that the resultant current flowing is given by the portion between the curve of Fig. 8 and the line bt taken as a base. lBy a reversal of terminals in the circuit we have the equivalent of showing the resultant current with bt as base line. This current is now applied to a second modulator M2 into which there is also fed a carrier frequency fz which may be dilerent from the carrier frequency f1 but would preferably be of the same value. To emphasize the point that there has been a turnover of the detector component, the terminals from the detector D1 are shown as crossed over. The output of the modulator M2 is now impressed on the single side-band filter F2 and the output of the filter is then transmitted to the receiving terminal for detection by the detector Dz. It will be recognized that ordinarily the output of the detector Dz will be of substantially the same form as the current wave impressed on the modulator Mz and if this were used to control a light valve recording the picture, the picture itself would then be reversed, appearing light where the original picture was dark. It is necessary, therefore, to bring about a second turnover to compensate for the turnover at the detector D1. Here again this is readily accomplished by means of the battery I3 poled in the correct direction to bring about the turnover, and again to emphasize that such a turnover occurs, the terminals from the detector D2 are crossed over.

From the description thus given of this system it is seen that the distortion introduced by the filter F1 is compensated for by the distortion introduced by the filter F2 and that this becomes possible as a result of the turnover after detection at D1, and further, thata ys'econdturnover at VD2 yor some equivalent point is thus necessar to compensate for the first turnover. l

What is claimed is: 1. Insingle side-band telegraph signaling, the method for compensating for .quadrature distortion associated with the single side-band and resulting inr bias of marking tospacingiwhich .consists in passing a signal ycontrolled Wave through a filter for isolating a single side-band thereof, transmitting the said side-band, detecting and receiving it at the receiving end and applying thereat a balancing marking to spacing biasing effect equal to the average biasing effect due to the quadrature component.

A2. In single side-band carrier Vfrequency signaling in which the signal has Abeen passed through a filter to pass one side-band, the method of reducing distortion due to the filter which consists in detecting the message, producing reversed signals thereof, controlling a second carrier frequency by .said reversed signals, filtering the resulting signal-controlled wave, passing it through a second filter passing one side-band, land transmitting the resulting side-band.

3. In single side-band telegraph signaling, the method of compensating for quadrature distortion associated with the single side-band and resulting in bias vof marking to spacing which consists in filtering a signal-controlled band for isolatingv a single kside-band thereof, detecting the message, producing reversed signals thereof, controlling a second carrier wave by said reversed signals,ltering the resulting signal-controlled wave in a similar ltcr and transmitting the resulting side-band. 1

4.` Ina single side-band carrier frequency signal transmission system, a transmission line, means at the transmitting end to modify a carrier wave by message, a filter for passing one side-band only of the resulting message-modied wave whereby marking to signal bias due to the quadrature component of the ltered wave is produced, means for transmitting the resulting side-band over said line, means at the receiving end to detect the received side-band and a `biasing means to balance out the average marking to spacing .bias due to vthe filter distortion. Y

5. In a single side-band Acarrier frequency signal transmission line tending to produce transmission bias in the signals, means at the transmitting Yend to modify a carrierfrequency waver by a message, a filter for passing one side-band only of the modified wave whereby marking to signal bias `due to the quadrature component/of the filtered wave is produced, means for trans-A mitting -said side-band over said line, means `at the receiving end to detect the received sideband and a marking to spacing biasing Ymeans to balance out the average bias due to the ilter distortion, and supplemental means to compensate for said transmission bias.

6. In-single side-band carrier frequency telegraph signaling characterized by quadrature distortion associated `with the single side-band and resulting in bias of marking vto spacing, means for yproducing a canrier frequency modulated with a message to form an upper and a lower side-band, a lter for passing one side-band only, ya detector, a relay operated thereby and so biased .as to produce reversed signals, means subject to -said relay for producing-a modulated carrier frequency, a second filter for passing lone side-band thereof only, and -rneans at the `receiving .end.for.-detecting the said last-named -side-band.

L7. TheV combination of claim 6 characterized vby the fact that the second modulated carrier frequency ris of thesame frequency as the first .carrier frequency.

`8. The combination of claim 6 characterized `by vthe fact that the rst-named detector has means to balance out the average bias effect of the quadrature component.

9.v The combinationof claim 6 characterize Vby the fact that the first-named detector has means to balance out the average bias effect of the quadrature component arising from the first filter, and means at the receiving detector to balance out-the average bias effect of the quadrature component arising from the second filter. 4

rv10. In a picture transmission system, means for deriving a marking-spacing current characteristic of the picture to be transmitted, means for modulating said current against a carrier frequency, a filter for transmitting one side-band only, a ydetecting and receiving device at the receiving station and means thereat to compensate for the quadrature component distortion arising from the filter, said last-named means consisting of a bias equal and oppos-ite to the marking-'spacing bias introduced by the said quadrature component distortion.

11. In a picture transmission system, means for deriving a marking-spacing current characteristic of the picture tobe transmitted, means for modulating said current against a carrier frequency, a filter for transmitting one sideband` only, means to compensate for'the` quadraturecomponent distortion arising from the filter,

said means comprising a detector and a relay for turning over the marking-spacing signal,

a second carrier frequency controlled bythe detected message, a second lter for transmitting one yside-'band only, vand means for impressing the resultant side-band on theY transmission circuit.

v 12. The combination of claim 11 characterizedby the fact that at the receiving station there isa second detecting circuit controlling ka light valve and a biasing'means associated with each detector adjusted to balance out the averagebiasing effect of the quadrature component at its'po'int inthe transmission circuit.

13. VIn la picture transmission system means for deriving a variable current characteristic of the picture to rbe transmitted, means for causing said current tocontrol a carrier frequency, a filter for' transmitting one side-band only, means to compensate for the'quadrature component distortion arising from the lter, said means comprising a detector and a circuit for turning ov-er the vdetected current, means for causing the turned over message to control, 

